Tensile Stress Evolution Research Articles

Solid-state reactions and stress evolutions between SnAg and Ni(P) thin films

Phase transformations in SnAg–Ni80P20 films were studied ex situ in parallel with in situ measurements of the corresponding transformation-induced stresses. Layered formation of Ni3Sn4 and Ni3P phases at an early stage of a reaction between SnAg and Ni80P20 films resulted in a tensile stress similar to the stress evolution in Sn–Ni80P20 films, despite the additional formation of Ag3Sn phase. Ag3Sn phase did not significantly affect the degree of stress evolution because of its islandlike and sporadic formation on the top surface of the Ni3Sn4 layer. Isothermal annealing showed that compressive stress, which was induced by the dominant formation of Ni3Sn4, developed after an initial evolution of tensile stress.

Local Corrosion of the Oxide Passivation Layer during Cu Chemical Mechanical Polishing

In this article, we analyze the effect of complexing agents in Cu chemical mechanical polishing slurry on the formation of oxide and the evolution of stress. The passivation property and surface morphology of the oxide on the surface showed significant differences depending on the kind of complexing agent. Oxalic acid showed fast oxide formation with poor passivation performance, and this caused large tensile stress evolution over 250 MPa in the Cu film. The synergetic effect of stress evolution and temperature increase due to the friction during the polishing caused severe pitting of the Cu surface after polishing in oxalic-acid-based slurry.

Semicircular bend testing with split Hopkinson pressure bar for measuring dynamic tensile strength of brittle solids

We propose and validate an indirect tensile testing method to measure the dynamic tensile strength of rocks and other brittle solids: semicircular bend (SCB) testing with a modified split Hopkinson pressure bar (SHPB) system. A strain gauge is mounted near the failure spot on the specimen to determine the rupture time. The momentum trap technique is utilized to ensure single pulse loading for postmortem examination. Tests without and with pulse shaping are conducted on rock specimens. The evolution of tensile stress at the failure spot is determined via dynamic and quasistatic finite element analyses with the dynamic loads measured from SHPB as inputs. Given properly shaped incident pulse, far-field dynamic force balance is achieved and the peak of the loading matches in time with the rupture onset of the specimen. In addition, the dynamic tensile stress history at the failure spot obtained from the full dynamic finite element analysis agrees with the quasistatic analysis. The opposite occurs for the test without pulse shaping. These results demonstrate that when the far-field dynamic force balance is satisfied, the inertial effects associated with stress wave loading are minimized and thus one can apply the simple quasistatic analysis to obtain the tensile strength in the SCB-SHPB testing. This method provides a useful and cost effective way to measure indirectly the dynamic tensile strength of rocks and other brittle materials.

Stress in sputter-deposited Cr films: Influence of Ar pressure

We studied the tensile stress and grain-width evolution in sputter-deposited Cr films with thickness from 20nm to 2.7μm. Films were deposited in an industrial Hauzer 750 physical vapor deposition machine at 50–80°C. The films exhibited a columnar microstructure. A power law behavior of the tensile stress as well as of the average grain width with thickness was observed. Both power exponents were strongly dependent on the Ar pressure during deposition. The power exponent α for stress varied from 0.26 to 0.79 for the range of Ar pressures used (5×10−3–2×10−2mbar). The mechanism of tensile stress generation is the shrinkage of the grain boundaries. Assuming the same shrinkage of the grain boundaries all through the layer, the stress and the grain width would be inversely proportional. Indeed, the grain width followed the same power law as the stress at low Ar pressure [α=0.3(1)], but not at high Ar pressure [α=0.58(3)]. Transmission electron microscopy showed the formation of numerous voids. At higher Ar pressure the void fraction is significantly higher than at low pressure, thereby diminishing stress generation.

Residual stresses in sputter-deposited copper/330 stainless steel multilayers

The evolution of residual stresses as a function of bilayer period from 10nmto1μm in sputter-deposited Cu/330 stainless-steel (SS) multilayered films is evaluated by the substrate curvature technique. The multilayer stress evolution is compared with residual stresses in single layer Cu films and single layer 330 SS films, also measured by substrate curvature technique, with respective film thicknesses varying from 5to500nm. Both single layer and multilayer films exhibit high tensile residual stresses that increase with decreasing layer thickness, but are found to be lower than the respective yield strengths. The intrinsic tensile residual stress evolution with film thickness is explained using the island coalescence model. The difference between the multilayer residual stress and the average residual stresses in single-layered Cu and 330 SS films is interpreted in terms of interface stress.

Chemistry-induced intrinsic stress variations during the chemical vapor deposition of polycrystalline diamond

Intrinsic tensile stresses in polycrystalline films are often attributed to the coalescence of neighboring grains during the early stages of film growth, where the energy decrease associated with converting two free surfaces into a grain boundary provides the driving force for creating tensile stress. Several recent models have analyzed this energy trade off to establish relationships between the stress and the surface∕interfacial energy driving force, the elastic properties of the film, and the grain size. To investigate these predictions, experiments were conducted with diamond films produced by chemical vapor deposition. A multistep processing procedure was used to produce films with significant variations in the tensile stress, but with essentially identical grain sizes. The experimental results demonstrate that modest changes in the deposition chemistry can lead to significant changes in the resultant tensile stresses. Two general approaches were considered to reconcile this data with existing models of stress evolution. Geometric effects associated with the shape of the growing crystal were evaluated with a finite element model of stress evolution, and variations in the surface∕interfacial energy driving force were assessed in terms of both chemical changes in the deposition atmosphere and differences in the crystal growth morphology. These attempts to explain the experimental results were only partially successful, which suggests that other factors probably affect intrinsic tensile stress evolution due to grain boundary formation.

Whisker Formation in Sn Coatings on Cu

ABSTRACTIn the microelectronics industry, Sn is often electroplated as a protective layer on Cu conductors. Pure Sn layers on Cu develop whiskers that can cause component failures and have even been implicated in the loss of several satellites. Alloying Sn with Pb suppresses whisker formation, but the push towards Pb-free processing will make this unacceptable in the future. To understand the driving forces and mechanisms of whisker formation on pure Sn, we are measuring the kinetics of stress evolution and intermetallic formation in Sn/Cu layers. By using thin films of Sn and Cu, we can monitor the stress evolution in real time using wafer-curvature based techniques. Preliminary results of stress evolution in vapor-deposited films are presented showing the evolution of tensile stress in the Cu layers and compressive stress in the Sn layers.

Intrinsic tensile stress and grain boundary formation during Volmer–Weber film growth

The finite element calculations in this letter present a detailed picture of tensile stress evolution during island coalescence in polycrystalline films. Earlier models suggest that these stresses are produced only when islands initially coalesce. In contrast, our model predicts that stress evolution is an inherent part of the growth process when two neighboring surfaces grow together to form a grain boundary. This view of coalescence stress is also consistent with experimental observations.

Intrinsic stress evolution in aluminum nitride thin films and the influence of multistep processing

Curvature measurements were used to investigate intrinsic stresses in AlN thin films grown by molecular beam epitaxy. In situ stress evolution experiments show that tensile stress is generated at the time where individual islands coalesce into a continuous film, and for some time after coalescence. Compressive stress is also generated both before and after coalescence. Introducing an intermediate annealing stage appears to reduce tensile stress evolution after coalescence, ultimately increasing the final intrinsic compressive stress. The maximum reduction in the tensile intrinsic stress is obtained when the sample is annealed at roughly the time where the islands coalesce into a continuous film.

Intrinsic residual stresses in metal films synthesized by energetic particle deposition

Residual stresses are intrinsic to all physical vapor deposited coatings. Bombardment with energetic particles (ions or neutrals) has a strong influence on both the sign and the magnitude of intrinsic stresses. For example, stresses can change from large tensile to large compressive with increasing particle bombardment. We develop a model for the evolution of tensile residual stress with increasing bombardment based on the hypothesis that the tensile stresses originate due to the attractive interatomic forces between the coalescing crystallites. The model predictions are shown to compare favorably with experimental results.

Tensile stress evolution during deposition of Volmer–Weber thin films

A simple model is presented that predicts the kinetics of tensile stress evolution during the deposition of thin films that grow by the Volmer–Weber mechanism. The generation of a tensile stress was attributed to the impingement and coalescence of growing islands, while concurrent stress relaxation was assumed to occur via a microstructure-dependent diffusive mechanism. To model the process of island coalescence, finite element methods were employed and yielded average tensile stresses more consistent with experimental observations than those predicted using previously reported analytical models. A computer simulation was developed that models the process of film growth as the continuous nucleation of isolated islands, which grow at a constant rate to impinge and coalesce to form a continuous polycrystalline film. By incorporating the finite element results for stress generation and a microstructure-dependent stress relaxation model, the simulation qualitatively reproduced the complex temperature-dependent trends observed from in situ measurements of stress evolution during the deposition of Ag thin films. The agreement includes simulation of the decreasing stress relaxation rate observed during deposition at increasing temperatures.

The role of oxygen in the intrinsic tensile residual stress evolution in sputter-deposited thin metal films

The intrinsic tensile stress and oxygen content in thin Cr films sputter deposited at high argon pressure are investigated as a function of negative substrate bias. With increasing bias from 0 to −100 V, the tensile stress increases to a maximum while the oxygen content decreases. At bias higher than −100 V, the tensile stress decreases but no significant change is observed in the oxygen content. A similar trend in stress evolution with bias is also observed in Cr films capped with Al to protect from postdeposition oxidation. It is concluded that, for a given deposition condition, the incorporation of oxygen may lower the magnitude of tensile stress without affecting the trends in stress evolution with increasing substrate bias. These effects are discussed using a model that correlates the film microstructure with long range attractive forces predicted from interatomic potentials.

Residual stresses in polycrystalline Cu/Cr multilayered thin films

Residual stresses in sputter-deposited Cu/Cr multilayers and Cu and Cr single-layered polycrystalline thin films were evaluated by the substrate curvature method. The stresses in the multilayers were found to be tensile and to increase in magnitude with increasing layer thickness (h) to a peak value of ∼1 GPa for h = 50 nm. For h > 50 nm, the residual stress decreased with increasing h but remained tensile. The same trends were observed in single-layered Cu and Cr thin films, except that the maximum stress in Cu films is 1 order of magnitude lower than that in Cr. Transmission electron microscopy was used to study the microstructural evolution as a function of layer thickness. The evolution of tensile growth stresses in Cr films is explained by island coalescence and subsequent growth with increasing thickness. Estimates of the Cr film yield strength indicated that, for h ≥ 50 nm, the residual stress may be limited by the yield strength. Substrate curvature measurements on bilayer films of different thicknesses were used to demonstrate that a non-negligible contribution to the total stress in the multilayers arises from the interface stress.

高分子材料 一軸延伸により熱成形したポリ塩化ビニル板の最適強度の検討

Optimum strength of polyvinyl-chloride (PVC) sheet thermoformed by uniaxial stretching was studied by mechanical test, thermally stimulated deformation recovery and structural observation using polarized light. In spite of tensile stress evolution during rapid cooling of thermoformed sheets, their deformation recovery behavior at reheating showed no evidence of storage of strain energy probably lessening static strength of thermoformed sheets. Sheets thermoformed at 100°C had quite high ductility presumably due to the peculiar crystalline structure still left in PVC after thermoforming. Sheets thermoformed at 130°C showed much less ductility and was found to have no crystalline structure.

Via Electromigration Lifetime Improvement of Aluminum Dual-Damascene Interconnects by Using Soft Low-k Organic SOG Interlayer Dielectrics

AbstractThe via electromigration(EM) reliability of aluminum(Al) dual-damascene interconnects by using Niobium(Nb) new reflow liner is described. It has been found that the via EM lifetime was improved by introducing low-k organic spin on glass(SOG)-passivated structure than the conventional TEOS-SiO2/SiN-passivated structure. Higher EM activation energy of 1.08 eV was obtained for the SOG-passivated structure than the conventional TEOS-passivated structure of 0.9 eV, even though no significant Al micro-crystal structure difference was found for both structures. It has been turned out that the low-k SOG material has the 1/7 Young's modulus (8 GPa) of TEOS-SiO2 (57 GPa) or thermal SiO2(70 GPa). The small Young's modulus means that SOG is more elastically deformable and/or softer than TEOS or thermal SiO2. This elastic deformation of the low-k SOG could retard the tensile stress evolution due to the Al atom migration near the cathode via, and elongated the time until the Al interconnect tensile stress exceeds the critical stress value for void nucleation. It has been concluded that the small-RC and reliable multi-level Al interconnect can be realized by the Nb-liner reflow-sputtered process with soft and low-k SOG dielectric materials.

Evolution of tensile residual stress in thin metal films during energetic particle deposition

Physical-vapor-deposited thin metal films often exhibit tensile residual stresses. We studied the stress evolution in thin Cr films and found that increasing bombardment with energetic particles (atoms or ions) at low energies leads to an increase of tensile stress to a maximum followed by a rapid decrease. Microstructural characterization by transmission electron microscopy revealed that two different microstructures are observed for the same level of tensile stress: films processed at low bombardment had columnar porosity while no porosity was observed in films processed at higher bombardment. The observed stress evolution is interpreted by considering how the mean interatomic distance (and hence the force) in the intercolumnar regions is modified by energetic particle bombardment.

Stress and Dopant Activation in Solid Phase Crystalized Si Films

ABSTRACTDefect creation mechanisms during solid phase crystallization (SPC) of Si thin films were investigated with PECVD amorphous precursor samples produced with various deposition temperatures and thicknesses. These precursor films were implanted with dopant and then crystallized to obtain both SPC and dopant activation. The doping efficiency was found to decrease with the tensile stress level as measured by Raman shift. The stress shows a decrease as the precursor deposition temperature and thickness are lowered. Furthermore, a lower level of stress is induced by rapid thermal annealing when the annealing temperature is high enough to soften the glass substrate on which the films were deposited. We show that by control of stress during the SPC step, intragrain defect density can be lowered and electronic quality of the resulting polycrystalline Si films can be improved. Based on these observations, we propose the following tentative model to explain the defect creation: during SPC, tensile stress evolution is considered to result from the volumetric contraction of Si film when it transforms from the amorphous to crystalline phase. This contraction is retarded by the substrate, which imposes a tensile stress on the film. A high level of stress leads to formation of structural defects inside the grains of the resulting polycrystalline material. These defects trap carriers or complex with the dopant reducing doping efficiency.

Stress evolution in ion assisted thin metal film deposition

Abstract Thermal atom deposited thin films are usually under dense, voided and in a state of tensile stress. Simultaneous ion bombardment during film growth can increase density, modify tensile stress and even generate compressive stress. The present analysis evaluates a number of collisionally motivated densification processes and predicts the dependence of tensile stress evolution, in each case, upon the parameters of ion flux density and ion energy and atomic deposition rate. A further set of models for the development of compressive stress, associated with dislocation loop and dislocation formation either by direct interstitial loop punching from collision cascades or by agglomeration and capture of freely migrating interstitials, are also considered. More complex dependences upon processing parameters and possible dependence upon film thickness are deduced.